1. Field of the Invention
The present invention relates to an optical system for an optical device with a stabilized image which moves an image of an object to keep it within a frame even if a still camera, video camera or telescope is vibrated, or an optical device for tracking fine movement of a object without panning the entire optical device.
2. Related Background Art
In an image stabilizing compensation optical system in a prior art imaging optical system, a refraction element, for example, a variable apex angle prism P is arranged at an end of an imaging optical system S closer to an object as shown in FIG. 1, and an apex angle .theta. is varied in accordance with a swing of the device to deflect an image. Such an optical system is disclosed in U.S. Pat. No. 3,212,420. However, because of a color dispersion effect of a prism material, a magnification chromatic aberration proportional to a focal distance f of the imaging optical system S and a dispersion of the prism P and a focusing performance is degraded.
For example, when a reference wavelength .lambda..sub.0 deflected by the prism P is directed to the imaging optical system S parallel with an optical axis of the imaging optical system S, other wavelengths .tau..sub.n (n=1, 2, 3, . . . ) are directed to the imaging optical system S provided rearward at exit angles .pi..sub.n (n=1, 2, 3, (angles between the optical axis and incident directions) determined by the dispersion of the material of the prism P. Accordingly, a magnification chromatic aberration .DELTA.y.sub.n (n=1, 2, 3, . . . ) at the wavelength .lambda..sub.n created by the prism P is represented by EQU .DELTA.y.sub.n =f.multidot.tan .tau..sub.n (n=1, 2, 3, . . . )(1)
For example, assume that a handy video camera is used in hand and an inclination (compensation deflection angle) of a deflected image due to a shaking of the device relative to the optical axis of the imaging optical system is 3.degree.. The focal distance of the imaging optical system is 50 mm, and a refraction type variable apex angle prism is made of silicone rubber having refraction coefficients for d-line, g-line and c-line of n.sub.d =1.4059, n.sub.g =1.4156 and n.sub.c =1.4035, respectively. The magnification chromatic aberration .DELTA.y.sub.n represented by the formula (1) is .DELTA.y.sub.g =-62 .mu.m and .DELTA.y.sub.c =15 .mu.m with respect to the d-line.
In order to maintain a focusing performance, it is desirable that a range of the magnification chromatic aberration is less than 10-20 .mu.um. Thus, in the prior art system, the compensation angle should be limited to less than 30.degree., or the focusing performance must be sacrificed.
On the other hand, Japanese Patent Application Laid-open No. 7416/1982 proposes to vary apex angles of a plurality of variable apex angle prisms made of media having different dispersions in order to prevent the chromatic aberration. This method, however, requires complex angle control or is limited to a proper combination of media.